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Underwater Acoustic Vector Sensor Development and Applications
High Directional Sensitivity in Low-Frequency Ranges Make New Sensors a Potential Alternative to Conventional Hydrophones

Dr. Tuncay Akal
SUASIS Underwater Systems Technology R&D Ltd.
Kocaeli, Turkey

Dr. Hans-Elias de Bree
R&D Director
Microflown Technologies
Arnhem, The Netherlands

Dr. Yuebing Wang
Hangzhou Applied Acoustic Research Institute
Hangzhou, China

An underwater acoustic wave is a propagating energy that causes a disturbance in the ambient pressure, creating a volumetric motion where the rate of this motion is particle velocity. Although the amplitudes of acoustic pressure and particle velocity are related, particle velocity is a vector that carries directional information regarding the propagating acoustic energy. While a single hydrophone can only measure scalar acoustic pressure, an acoustic vector sensor (AVS) is capable of measuring all three components of acoustic particle motion: acoustic pressure, velocity and direction.

In recent years, extensive research has been conducted on the design of vector sensors, which are mainly designed on gradient or inertial concepts. Gradient sensors measure the gradient of the acoustic pressure in one direction to obtain the velocity and acceleration, whereas inertial sensors directly measure the acoustic medium's motion in a specific direction.

As an AVS measures the underwater acoustic field in 3D, it allows users to measure low frequencies at a single point in space without requiring the long arrays with spatially separated, pressure-sensitive hydrophones traditionally used for this application. Because of the directional, low-frequency response of these sensors, there are many applications when integrated into surveillance systems at sea.

Hydroflown and VHS Overview
This article will discuss two different AVS approaches: the Hydroflown and the acoustic vector hydrophone sensor (VHS). SUASIS Underwater Systems Technology R&D Ltd. led the research and development as well as applications development on these separately developed projects, collaborating with Microflown Technologies on the Hydroflown and with the Hangzhou Applied Acoustic Research Institute (HAARI) on its VHS.

The Hydroflown, a microelectromechanical system-based AVS that measures particle velocity and its direction in nanoscales, is now going through tests for static applications. Research is proceeding to place the Hydroflown inside towed arrays, and its acceleration and flow noise characteristics are in development. Versions of the HAARI VHS will be available in the market within this year through the Underwater Silk Road program, a Turkish-Chinese collaboration between SUASIS and HAARI.

Both sensors are ready for static applications. Since May, two versions of VHS have been ready for sale, whereas Hydroflowns for static applications are undergoing tests as a module for a passive detection system. Hydroflowns are not presently for sale as single sensors, but some of the static application units will be on sale toward the end of the year.

Hydroflown Acoustic Particle Velocity Sensor
The Hydroflown, based on a Microflown Technologies air sensor, is able to measure the particle velocity in a bandwidth of one hertz to 20 kilohertz. The Microflown measures particle velocity using two parallel platinum wire resistances. When voltage is applied across the wire terminals, the wires heat up to 300° C. An acoustic wave propagating perpendicular to the wires results in a temperature difference between the wires. The upstream wire will cool down more compared to the downstream wire due to convective heat transfer, which will result in a change in the resistance of the wires. The particle velocity is proportional to the voltage change induced by the change in the resistance.

In autumn 2008, SUASIS and Microflown Technologies began development to adapt the particle velocity sensors for underwater applications with support from the EUROSTARS Programme, a European Union-funded project dedicated to research and development-focused small and medium enterprises. Although the working principle of the Hydroflown sensor is similar to the Microflown sensor, the Hydroflown is designed to work underwater and is required to be encapsulated in an acoustically transparent package filled with nonconducting fluid that has a high boiling temperature.

To adapt the Microflown sensors for underwater use, the microelectromechanical systems design and packaging were completely modified, and Hydroflown prototypes were produced for various tests and calibrations in laboratories, lakes and calibrations tanks in both the Netherlands and in Turkey. The sensitivity and directivity of the Hydroflown sensors were measured in the laboratory using a standing wave tube. In a standing wave tube, the sound pressure and particle velocity are related in a relatively simple manner, making it possible to compare the pressure-sensitive and particle-velocity-sensitive sensors. A small motor-driven rotating disk with a Hydroflown mounted to the standing wave tube helped the authors obtain a receiving sensitivity beam pattern of the Hydroflown.

The Hydroflown's directional capability comes from its dipole behavior. Each Hydroflown is designed to be sensitive only in one direction while being insensitive in the other directions. The resultant beam pattern for this type of sensor is figure-eight shaped. The vector sensor's beam patterns show cosine and sine responses, a type of beam pattern that is essential for directional signal processing. The operational bandwidth of the present Hydroflown prototype is between five hertz and two kilohertz. SUASIS and Microflown Technologies are developing new microelectromechanical units to increase the frequency range to 10 kilohertz.

Acoustic Vector Hydrophone Sensor
The HAARI began VHS development in 2006 for directional measurements in calibration tanks. Research and development has continued since, and recent improvements have allowed the VHS to function as a directional receiver. In cooperation with SUASIS, new applications are being developed.

The spherical VHS is made of three pairs of accelerators set in three orthogonal directions. These sensors and their preamplifiers are fixed onto a base block and sealed into a polymer ball. To keep the VHS in a suitable orientation, three pairs of fixing rings are attached to the outside of the polymer ball. Power to preamplifiers is supplied using phantom power, a technique where electric power and signal communication take place over the same wires. With this technique, the number of wires is reduced to six and cables up to hundreds of meters in length can be used. To continue this article please click here.

Dr. Tuncay Akal is the co-founder of SUASIS Underwater Systems Technology R&D Ltd. For more than 33 years, he was principal and senior scientist at the NATO Undersea Research Centre (NURC), where he led research and development projects. Since leaving NURC, he has worked on the development of vector sensors and their applications.

Dr. Hans-Elias de Bree, co-founder of Microflown Technologies, graduated from the University of Twente in the Netherlands. He obtained his master's degree, and after inventing the Microflown, got his Ph.D. He was appointed as a professor at the HAN University of Applied Sciences' Arnhem School of Automotive Engineering.

Dr. Yuebing Wang joined Hangzhou Applied Acoustic Research Institute in 1990. He was appointed as a member in International Electrotechnical Commission TC87 in 2001. His particular research interests are underwater acoustics and medical ultrasound, especially the designing of new hydrophones and inventing measurement methods.

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